Scientists from the John Innes Centre have discovered an unusual and novel role for the plant hormone auxin in the development of the female organs of the flower.

As they develop, the structures of many organisms begin as a ball of similar cells with radial symmetry, which then develop bilateral symmetry as the cells differentiate and the organ matures. Until now, evidence of post-embryonic organ development in the opposite direction – from a bilaterally symmetrical form to a radial one – has only been observed in the sea urchin.

However, plant scientists Professor Lars Østergaard and Laila Moubayidin from the John Innes Centre in Norwich have discovered that tissue at the tip of the gynoecium – the structure that forms the female reproductive organ of a flower – also goes through a bilateral to radial transition. The resulting radial structure makes up the style of the gynoecium. Using the weed Arabidopsis thaliana in their experiments, the researchers found this unusual transition to be controlled by two genes directly affecting auxin distribution.

Professor Østergaard said: “In numerous cases, auxin has been characterised for its role in providing identity to cells, so that they differentiate differently from their environment. In this case however, the early gynoecium structure has a bilateral pattern of cells along its entire length. We have found that expression of two genes controlling the distribution of auxin programs different cell types at the tip of the gynoecium to become homogeneous, setting up a radial pattern.”

Having a radial style is crucial for effective fertilisation since the tubular shape supports the growth of pollen tubes into the gynoecium. These carry sperm from the male parts of the flower towards the female gametes, ultimately leading to the development of a new plant embryo.

Dr Moubayidin said: “This is really interesting from an evolutionary point of view. Angiosperms such as Arabidopsis thaliana make up over 90% of all plants on earth, yet they diversified from simpler plants relatively recently in history. Charles Darwin called this rapid domination ‘the abominable mystery’. Perhaps, when the ability to develop a radial style evolved, this was a key evolutionary event explaining why angiosperms are so reproductively successful.”